1. Field of the Invention
The invention relates to a sense amplifier for a memory cell, and more particularly, a sense amplifier for a memory cell that has a fast sensing speed.
2. Description of the Prior Art
One common way to determine the bit data stored in a memory cell is to compare a cell current of the memory cell with a reference current. For example, if the cell current of the memory cell is greater than the reference current, the bit stored in the memory cell may be determined to be 0. If the cell current of the memory cell is smaller than the reference current, the bit stored in the memory cell may be determined to be 1.
FIG. 1 shows a sense amplifier 100 according to prior art. The sense amplifier 100 includes a cell current generator 110, a reference current generator 120, and an output buffer 130. The cell current generator 110 is coupled to a memory cell 10 and the cell current generator 110 is used to generate a cell current Icell according to the current outputted from the memory cell 10. The reference current generator 120 is used to generate a reference current Iref. The output buffer 130 is coupled to a node N1. The node N1 is coupled to both the cell current generator 110 and the reference current generator 120. When using the sense amplifier 100 to determine the bit data, the cell current generator 110 can output the cell current Icell going into the node N1 and the reference current generator 120 can output the reference current Iref going out of the node N1. Consequently, when the cell current Icell is greater than the reference current Iref, the voltage level of the node N1 will be raised up to a higher voltage level by the cell current generator 110, and when the cell current Icell is smaller than the reference current Iref, the voltage level of the node N1 will be pull down to a lower voltage level by the reference current generator 120.
According to the sense amplifier 100, the bit data stored in the memory cell 10 can be determined when the cell current Icell or the reference current Iref finally causes the voltage level of the node N1 to be greater or lower than a threshold value. Namely, the time required for determining the bit data is significantly dependent on the difference between the cell current Icell and the reference current Iref. For example, if the cell current Icell is only slightly greater than the reference current Iref, it will take much more time for the cell current generator 110 to raise the voltage level of the node N1 to be greater than the threshold value before one can certainly determine the bit data, which is very inefficient for determining the bit data. Thus, how to reduce the time of determining the bit data while stabilizing the time to determine the bit data has become a critical issue to be solved.